The present invention relates generally to laser projectors and, more particularly, to fiber optic projection devices.
Video and other types of projectors are required for a wide variety of applications. While a variety of projection devices have been developed, laser projection devices are currently being developed in order to take advantage of the crisp, bright images exhibiting high color saturation that are provided by laser illumination. In the defense industry, for example, laser projectors can be utilized to create images for use in command centers, electronic cinemas, flight simulators and virtual sand tables in which an image of some portion of the terrain is created.
A typical laser projector includes a red laser, a green laser and a blue laser for emitting red, green and blue light, respectively. A laser projector also includes an opto-mechanical or acousto-optical scanning system for imaging the red, green and blue light onto a screen or other display to thereby produce a full color image.
Various lasers have been utilized in the past to generate the red, green and blue light. For example, laser sources have included visible ion lasers, frequency doubled solid state micro-lasers and electron beam pumped semiconductor lasers. Even though laser projectors have been developed with a wide variety of laser sources, each of these laser projectors suffer from at least some limitations. For example, lasers are notoriously inefficient in converting electrical input power to output optical power. As such, at least some of the conventional laser projectors cannot economically be scaled up so as to provide red, green and blue light having relatively high power levels.
In addition, the images created by at least some laser projectors disadvantageously exhibit laser speckle in instances in which the laser sources produce signals insufficient spectral width. In this regard, laser speckle is intrinsic to narrow-band laser sources in which the speckle manifests itself as fine grain intensity fluctuations in the resulting image. The specific distribution of the intensity fluctuations are rearranged in a random fashion whenever the viewer""s head moves slightly in a lateral or vertical direction. The visual effects of speckle detract from the quality of the resulting image and reduce the resolution of the resulting image. Thus, high resolution display systems generally desire to minimize, if not eliminate, laser speckle.
The performance of a laser projector is also compromised if the laser sources do not permit the wavelength of the signals emitted by the laser sources to be tuned somewhat in order to adjust the range of hues in the resulting image. In addition the definition of the images projected by a laser projector will be limited if the intensity of the output of the red, green and blue lasers cannot be modulated over relatively wide bandwidths.
While laser projectors having the aforementioned laser sources offer at least some of the features desired of a laser projection system, conventional laser projectors are each limited in one respect or another. Thus, it would be advantageous to provide an improved laser projector that produces crisp, bright images that exhibit high color saturation while addressing each of the other aforementioned features.
A fiber optic projection device is therefore provided that includes red, green and blue fiber optic sources that efficiently convert an electrical input into optical power by emitting signals having red, green and blue colors, respectively. As such, the fiber optic projection device can generate signals having red, green and blue colors that are scalable to relatively high power levels while avoiding laser speckle. Accordingly, the images generated by the fiber optic projection device of the present invention should be crisp, bright and should exhibit high color saturation.
Each of the red, blue and green fiber optic sources includes at least one fiber amplifier for emitting an amplified signal of a predetermined wavelength. In order to provide signals having red, green and blue colors that are scalable to high power levels, each fiber optic source of one advantageous embodiment includes a plurality of fiber amplifiers and a combiner for combining the amplified signals emitted by each of the plurality of fiber amplifiers. Regardless of the number of fiber amplifiers, each fiber optic source also includes a frequency altering device, such as a frequency doubler, for altering the frequency of the amplified signals to thereby produce a signal having the respective color, such as red, green or blue. The fiber optic projection device of the present invention also generally includes at least one modulator for individually modulating the intensity of the signals having the red, green and blue colors. Further, the fiber optic projection device can include a projector for mixing the modulated signals having the red, green and blue colors in order to create a full color image.
In one embodiment, the fiber amplifiers of the blue fiber optic source are dual core fibers having an inner core comprised of neodymium doped silica. Likewise, the fiber amplifiers of the green fiber optic source can be dual core fibers having an inner core comprised of germanium doped silica that is further doped with either neodymium or ytterbium. In contrast, however, the red fiber optic source of one advantageous embodiment relies upon Raman gain to generate an amplified signal having a red color. Thus, the red fiber optic source of this embodiment includes a master oscillator for generating a first input signal and a red subharmonic oscillator for generating a second input signal having a Raman seed wavelength. The first and second input signals are selected and each fiber amplifier of the red fiber optic source is designed to provide Raman gain for signals having the Raman seed wavelength. For example, each fiber amplifier of the red fiber optic source can include a dual core fiber having an inner core comprised of ytterbium doped silica and further having a number of gratings written along the length thereof. The frequency altering device of the red fiber optic source of this advantageous embodiment therefore alters the frequency of the signals provided by the fiber amplifiers having the Raman seed wavelength to produce signals having the red color.
In one embodiment, the fiber optic projection device includes a plurality of modulators associated with respective fiber optic sources for receiving signals from the respective frequency altering device and for modulating the intensity of the signals. Alternatively, the fiber optic projection device can include a plurality of modulators associated with respective fiber optic sources for receiving signals from the respective fiber amplifiers and for modulating the intensity of the signals prior to delivering the modulated signals to the respective frequency altering device.
In an alternative embodiment, a master oscillator is provided that modulates the intensity of the signals having the red, green and blue colors. In this regard, it is noted that the frequency altering device preferably alters the frequency of the amplified signals that have a wavelength within a predetermined range of wavelengths, while rejecting signals having other wavelengths. Thus, the fiber optic projection device of this embodiment includes a plurality of modulated master oscillators for providing input signals to the fiber amplifiers of the respective fiber optic sources. Each modulated master oscillator includes first and second sources for alternately providing input signals having first and second wavelengths, respectively, to the respective fiber amplifiers. Each modulated master oscillator also includes a coupler for combining the input signals alternately provided by the first and second sources. The first source is preferably selected to provide input signals having the first wavelength that is within the predetermined range of wavelengths such that the frequency altering device alters the frequency of the amplified signals having the first wavelength. Conversely, the second source is preferably selected to provide input signals having the second wavelength that is outside of the predetermined range of wavelengths such that the frequency altering device rejects the amplified signals having the second wavelength.
Accordingly, by having the modulated master oscillator alternately provide input signals having the first and second wavelengths, the signals having the respective color can be modulated. As such, the modulated master oscillator of this advantageous embodiment of the present invention controls the duty cycle of the amplified signals having the first and second wavelengths and therefore controls the duty cycle of the signals emitted by the frequency altering device since the frequency altering device is only responsive to signals having the first wavelength. By alternately providing signals having the first and second wavelengths even though the frequency altering device is only responsive to signals having the first wavelength, however, the modulated master oscillator of this advantageous embodiment maintains relatively high saturation of the fiber amplifiers such that the fiber optic source generates signals having the respective color in a responsive and an efficient manner.
The fiber optic projection device of the present invention can therefore provide crisp, bright images that exhibit high color saturation in an efficient manner. By employing fiber optic sources that can include arrays of fiber amplifiers, the fiber optic projection device can produce signals having red, green and blue colors that are scalable to very high power levels. In addition, while the wavelength of the signals having the red, green and blue colors that are emitted by the fiber optic sources are somewhat tunable in order to adjust the range of hues that can be reproduced, the spectral width of the red, green and blue signals is sufficient to avoid laser speckle. In addition, the fiber optic projection device provides for video modulation of the signals having the red, green and blue colors over a wide bandwidth in order to provide for an image having high definition. Thus, the fiber optic projection device is suitable for a wide variety of applications including large screen projection systems and the like.